The present invention relates to user-actuated sensors and, more particularly, to a novel single electrode capacitance touchpad sensor and related system electronics.
Conventional capacitance touchpad devices typically require three separate electrodes, fabricated upon at least one surface of a supporting substrate. Typically, the touchpad electrode is fabricated upon the opposite side of a relatively thick dielectric substrate from the remaining two electrodes and may require an area of about one square inch. Even with relatively large touchpad electrode areas, the active capacitance of the sensor is typically less than 5 picofarads, whereby geometry variations, mask registration or parasitic capacitance problems will typically result in device capacitance variations of up to 2 picofarads, and create a device of extreme sensitivity to design process variations. Thus, the conventional touchpad sensor devices, being of a relatively high impedance design, exhibit significant performance differences between sensors, even when several sensors are arrayed upon the same substrate and fabricated simultaneously. It is known to alleviate parasitic capacitance coupling problems by utilizing relatively large device areas and relatively large amplitude of driving voltages. It is desirable to provide arrays of capacitive touchpad sensors having both a reduced sensor area, to provide for greater sensor density in the array, and with reduced requirements for the amplitude of the drive voltages, to provide greater safety to user personnel (contacting the touch electrodes upon which at least a portion of the drive voltage appears) and to reduce driving and sensing circuit complexity and attendant cost.